1. Technical Field
The present invention relates to a semiconductor having a superimposed doping layer of a plurality of element dopants and a method for producing the same.
2. Background Art
A doping technique for a semiconductor device is essential as a process for manufacturing a semiconductor device. As a method of doping, a δ-doping technique has been known which is used as an effective method for improving the operation speed of a high mobility transistor. The method is based on that provision of a doping layer at the level of an atomic layer near the hetero interface enables to inject carriers in the vicinity of the interface while avoiding alloy scattering due to a dopant. In a case of a compound semiconductor, the δ-doping technique can be realized easily. For example, in a compound semiconductor comprising group III and group V elements, since elements of groups III and group V are stacked alternately, dopant elements doped to the group III layer tend to be suppressed from diffusion by the stacking of the group V element layer.
In the δ-doping technique described above, when the dopant not of a single element but dopants of plurality of elements can be superimposed, the range of utilization as a process in the manufacture of the semiconductor device is extended. For example, the following existent examples can be mentioned for compound semiconductors. At first, in an example reported for compound semiconductors, an effect of making the interface of the δ-doping layer steep has been reported [“Compound semiconductor crystal and growing method thereof” (Patent Document 1)]
Alternatively, this can be utilized also for interaction between the state of spin and a conduction electron in a semiconductor. For example, Non-Patent Document 1 demonstrates the ferromagnetic order by confining conduction electrons using a hetero structure thereby strengthening interaction with the state of spin as shown in FIG. 17. In this example, while δ-doping for Be atoms is used for supplying conduction electrons to the vicinity of the hetero structure, δ-doping for Mn atoms as a spin source is not conducted. It can be predicted that the ferromagnetism order can be attained without preparing the hetero structure when doping for both of them can be superimposed.
Further, as a quantum information processing device for which δ-doping for such plurality of elements is effective, an entirely optically controlled solid quantum information processing device has been proposed (Non-Patent Document 2). In this case, as shown in FIG. 18, a quantum bit S1 and a quantum bit S2 are spaced apart by an inter-quantum bit distance DSTSS so that a base function W1 and a base function W2 are not overlapped and a rotational gate operation can be conducted by an optical operation and a magnetic field operation. Further, a control bit C is spaced apart from the quantum bit S1 or the quantum bit S2 by an inter-quantum bit and control bit distance DSTSC so as not to cause overlap to between base functions WCG to W1 or WCE to W2. It is theoretically predicted that a distance of about 7 to 10 nm is sufficient for the inter-quantum bit distance DSTSS and the inter-quantum bit-control bit distance DSTSC. When the conditions are satisfied, in a case where the wave function of the bit-control bit C is excited to the excited state WCE by the optical operation, an entangled state can be formed by interaction caused between the spin of the base functions W1 and W2 of the quantum bit S1 and the quantum bit S2 through the excited state. When the rotational gate operation and the entangle state can be attained, all quantum information processing operations can be conducted (Non-Patent Document 3).
While it is intended to utilize the δ-doping technique of conducting superimposed doping of a plurality of dopants also in an elemental semiconductor such as silicon, the attainment of the δ-doping technique itself is difficult different from the compound semiconductor. For example, FIG. 19 shows an existent example of δ-doping boron in silicon as an example of using 1-element dopant. In this case, boron is at first adsorbed by about 1-atomic layer to a silicon substrate SSI to deposit a boron adsorption layer BO and then silicon in an amorphous state is deposited near a room temperature. Then, when the temperature is elevated to about 500° C. and left as it is, an amorphous silicon layer AMR conducts solid phase epitaxial growing and transforms into a crystalline silicon layer EPSI aligned with the underlying substrate silicon. The boron layer is frozen in a sandwiched state to form a δ-doping layer BD as a doping layer at the level of an atomic layer. On the other hand, in a case of a plurality of elements, an adsorption layer of a plurality of elements is used instead of the boron adsorption layer but no actual example has yet been reported. This is because δ-doping using a plurality of elements as dopants by solid phase epitaxy was difficult so far in the elemental semiconductor.
As a significant problem, it can not be said that the solid phase epitaxy is an ideal method as a technique of conducting doping to a predetermined depth. The problem in the δ-doping technique is that a doping profile prevails in silicon solid phase epitaxy during solid phase epitaxy. Further, the process requires much labor and long time. The time required for the solid epitaxy is at the order of hours while this depends on the thickness of the amorphous layer to be stacked. No actual example has yet been reported for the δ-doping technique of conducting superimposed doping for a plurality of dopants and, accordingly, it may be considered that the amount of the doping profile prevailing in solid phase epitaxy is different depending on the kind of the dopant, although this is merely a matter of speculation. Accordingly, it may be considered that control for the concentration and the distribution width is extremely difficult in the multi-element δ-doping technique that utilizes the solid phase epitaxy.
However, there may be also considered another technique of ion implantation, as a technique for superimposed doping to a predetermined depth. In this case, the implanting depth and the doping profile are determined automatically depending on the acceleration energy of implanted ions and mass of the ions. Theoretically, a plurality of element dopants can be implanted each to an identical depth.
However, also the ion implantation technique is not ideal as a technique of doping to a predetermined depth. Primarily, in the ion implantation technique, the apparatus is expensive and the implantation depth and the doping profile are determined depending on the acceleration energy of implanted ions and the mass of the ions and, further, annealing is necessary for repairing lattice damages caused upon implantation. Theoretically, while superimposed doping for a plurality of dopants of implanting a plurality of elements into a substantially predetermined depth is possible, the distribution width can not be made identical. Further, in the technique intending to determine the doping position at a nanometer scale as in the δ-doping technique, error for positioning is not possible at the order of nanometer, and deviation of the depth is fetal.
Patent Document 1: JP-A No. 2002-25921
Non-Patent Document 1: A. M. Nazmul, S. Sugahara and M. Tanaka, Physical Review, Vol. B67 (2003) pp 241308
Non-Patent Document 2: A. M. Stoneham, A. J. Fisher and P. T. Greeland, “Optically driven silicon-based quantum gates with potential for high-temperature operation”, The Journal of Physics: Condensed, Matter, vol. 15, pp L447-L451, 2003
Non-Patent Document 3: N. Nielson and I. Chuang, “Quantum Computation and Quantum Information”, Chmbridge University Press, 2000